Why spend hundreds of dollars on a metronome? Most pros who play with a click (most pros...) program their clicks and run them through a laptop nowadays. You can get the exact sounds you want, special cues, whatever. If you want to not have to set up your laptop and prefer a metronome for when you're just playing around, I believe you should get something cheaper. If you still want to put a lot of money on a sub-par alternative, get the Dr. Beat.

I have been using a Rhythm Watch for three years and I like it. I especially like how you can tap a tempo into it, and it tells you how many BPM that is - great for solving band disagreements. I don't know about the BOSS, though.